There was also a Q&A with Anssi Hakkarainen from Nokia's Product Marketing Team a few weeks back, in which this question was posed:
Q: Will aftermarket covers with inductive charging be available/supported?
A: At the moment no plans for wireless charging shell for Nokia Lumia 620.
The signs aren't good, but the term 'at the moment' provides a bit of hope. Still, having made wireless charging an integral feature of the Lumia range, it seems strange not to offer it as an option on the 620.
Source:
Your Lumia 620 Questions Answered! : My Nokia Blog